This invention relates generally to optical elements, and more particularly to strengthening of optical elements which are transparent in both the visible and infrared range of optical wavelengths for use on a missile or another air flight vehicle.
As it is known in the art, there is a need for materials which are highly durable and which have substantial transparency in both the visible and infrared optical wavelength ranges. Applications for these materials include commercial systems, such as various vapor lamps and optical windows, as well as military systems, such as airborne optical imaging systems as found, for example, on a missile or the like.
With the aforementioned airborne optical imaging systems, such as those found on an infrared heat sinking missile, one or more optical elements, such as a window or a dome is mounted on an exterior portion of the missile to isolate the remaining optics of the imaging system from an external environment through which the missile is flown.
Thus, in addition to having the aforementioned substantial transparency in both the visible and infrared spectrums, these external elements must also have a particularly high degree of resistance to environmental exposures while having sufficient strength to protect the remaining components of the imaging system during operation of the missile.
Several materials have been identified as potential candidate materials for these applications. Amongst these materials are single crystal sapphire (cubic aluminum oxide) and aluminum oxynitride (AlON). These materials, in particular, have a very high degree of strength and are capable of having a relatively high degree of infrared transparency particularly over the wavelength range of about 0.2 microns up to about 5 microns. For infrared dome materials for use in medium wavelength infrared bands, i.e. the 3-5 micron band, AlON and single crystal sapphire are two superior candidate materials since each have relatively high degrees of strength and relatively high degree of infrared transmittance. Sapphire is currently the stronger of the two materials. However, sapphire is also extremely expensive to fabricate since it must be grown as a single crystal. Polycrystalline sapphire is not suitable for optical applications since polycrystalline sapphire has very high birefringence, thus making polycrystalline sapphire material unsuitable for imaging infrared optical energy.
It is known that compressive surface coatings can be provided over sapphire to increase fracture and flexural strengths. One approach has been to deposit SiO. Another approach has been to use a glass frit high in SiO.sub.2 content.
Although single crystal sapphire is a strong material, it is very difficult and expensive to fabricate. Moreover, for many applications it would also be desirable to provide domes and like elements fabricated from materials which are less expensive than sapphire but which have high levels of strength.